We quantified the relationship between atmospheric rivers (ARs) and occurrence and magnitude of extreme precipitation in western U.S. watersheds, using ARs identified by the Atmospheric River Tracking Method Intercomparison Project and precipitation from a high‐resolution regional climate simulation. Our analysis shows the potential of ARs in predicting extreme precipitation events at a daily scale, with Gilbert Skill Scores of ~0.2. Monthly extreme precipitation amount in west coast watersheds is closely related to AR intensity, with correlation coefficients of up to 0.6. The relationship between ARs and precipitation is most significant in the Pacific Northwest and California. Using a K‐means clustering algorithm, ARs can be classified into three categories: weak ARs, flash ARs, and prolonged ARs. Flash ARs and prolonged ARs, though accounting for less than 50% of total AR events, are more important in controlling extreme precipitation patterns and should be prioritized for future studies of hydrological extreme events.

Hydrologic exchange flows (HEFs) across the river‐aquifer interface have important implications for biogeochemical processes and contaminant plume migration in the river corridor, yet little is known about the hydrogeomorphic factors that control HEFs dynamics under dynamic flow conditions. Here, we developed a 3‐D numerical model for a large regulated river corridor along the Columbia River to study how HEFs are controlled by the interplays between dam‐regulated flow conditions and hydrogeomorphic features of such river corridor system. Our results revealed highly variable intra‐annual spatiotemporal patterns in HEFs along the 75‐km river reach, as well as strong interannual variability with larger exchange volumes in wet years than dry years. In general, the river was losing during late spring to early summer when the river stage was high, and river was gaining in fall and winter when river stage was low. The magnitude and timing of river stage fluctuations controlled the timing of high exchange rates. Both river channel geomorphology and the thickness of a highly permeable river bank geologic layer controlled the locations of exchange hot spots, while the latter played a dominant role. Dam‐induced, subdaily to daily river stage fluctuations drove high‐frequency variations in HEFs across the river‐aquifer interfaces, resulting in greater overall exchange volumes as compared to the case without high‐frequency flows. Our results demonstrated that upstream dam operations enhanced the exchange between river water and groundwater with strong potential influence on the associated biogeochemical processes and on the fate and transport of groundwater contaminant plumes in such river corridors.

Atmospheric rivers (ARs) can significantly modulate surface hydrological processes through the extreme precipitation they produce. However, there is a lack of comprehensive evaluation of ARs' impact on surface hydrology. This study uses a high‐resolution regional climate simulation to quantify the impact of ARs on surface hydrological processes across the western U.S. watersheds. The model performance is evaluated through extensive comparison against observations. Our analysis indicates that ARs produce heavy precipitation but suppress evapotranspiration. Snowpack ablates more during ARs, with higher air temperature and increased longwave radiation playing the primary and secondary roles, respectively. At the 0 °C to 10 °C temperature range, ARs increase the probability of snow ablation from 0.33 to 0.57. The runoff‐to‐precipitation ratio is primarily controlled by antecedent soil moisture, but it almost doubles in the northwestern watersheds due to the intensification of snow ablation during AR events. From the analysis of the relationship between the hydrological responses and different meteorological factors, precipitation, temperature, and radiation are identified as the key drivers that distinguish the hydrologic responses between AR and non‐AR events. Lastly, analysis of ARs and total runoff at annual scale and 1 April snowpack and winter precipitation shows that ARs explain 30% to 60% of the variability of annual total runoff and sharpen the seasonality of water resources availability in the west coast mountain watersheds.

SummaryThe overall objective of our studies was to specify an index describing the hydraulic force that fish experience when subjected to a shear environment. Fluid shear is a phenomenon that is important to fish. However, elevated levels of shear may result in strain rates that injure or kill fish. At hydroelectric generating facilities, concerns have been expressed that strain rates associated with passage through turbines, spillways, and fish bypass systems may adversely affect migrating fish. Development of fish friendly hydroelectric turbines requires knowledge of the physical forces (injury mechanisms) that impact entrained fish and the fish's tolerance to these forces. It requires up-front, pre-design specifications for the environmental conditions that occur within the turbine system; in other words, determining or assuming conditions known to injure fish will assist engineers in the design of a fish friendly turbine system. These biological specifications must be carefully and thoroughly documented throughout the design of an advanced turbine. To address the development of biological specifications, we designed and built a test facility where juvenile fish could be subjected to a range of shear environments and quantified their biological response.Test fish included juvenile rainbow trout, Oncorhynchus mykiss, spring and fall chinook salmon, O. tshawytscha, and American shad, Alosa sapidissima. Fish were exposed to a shear environment produced by a submerged jet over a range of exit velocities from 0 to 21.3 m/s (0 to 70 ft/s). They were introduced in either a headfirst or tailfirst orientation and to the edge of the jet stream (slow-fish-to-fastwater scenario) or within and upstream of the jet stream (fast-fish-to-slow-water scenario). Test fish were captured after leaving the shear environment and specific biological responses noted (i.e., injury and mortality). The behavior or reaction of fish in the shear environment was recorded on high-speed video cameras. Fluid velocities were measured in the jet with a Pitot tube and a Laser Doppler Velocimeter (LDV). Statistical tests were applied to the fish data to estimate the lowest observed effect level and no observed effect level, or the strain rate at which fish were not injured after being subjected to the shear environment.The Pitot tube provided mean velocity information in the axial direction. The mean-flow velocity measurements obtained using the Pitot tube were used to describe the jet centerline velocity and fluid strain rate. This report refers to the mean change in water velocity (u) over distance (y) as strain rate (e). We used strain rate as an index of the physical force that fish experience when subjected to the shear environment in our test facility. The rate of strain experienced by test fish varied from 0 cm/s/cm 1 to 1185 cm/s/cm, based on a spatial resolution of ∆y=1.8 cm. This interval was based on the minimum width of the salmonids tested. The values reported here are not equivalent to a strain rate computed at a finer scale resoluti...

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